The Focusrite driver you will have installed on your system is an ASIO driver. ASIO stands for ‘Audio Stream Input/Output’ and is a cross-platform, multi-channel audio transfer protocol developed by Steinberg. It allows recording software programs such as Ableton, Pro Tools, Cubase, Reaper etc to receive and output audio on multiple channels, as long as the hardware interface also has this ability. For example, when using a Scarlett 18i20 interface with Cubase on a supported version of Windows, Cubase will see all 18 inputs and 20 outputs.
Currently, Audacity does not support ASIO and so, therefore, runs off Windows inbuilt audio driver. As the Windows inbuilt audio driver only supports a maximum of two inputs and two outputs, regardless of the I/O count of your Focusrite interface, Audacity will only ever see the first two inputs and outputs of your device.
In short, you can use your Focusrite interface with Audacity, but you will only be able to use the first two inputs and outputs.
As OS X’s inbuilt audio driver (Core Audio) is able to support multichannel I/O, Audacity will see all inputs and outputs of any Scarlett / Saffire / Clarett interface.
Note that the way in which Audacity allows you to select which input you are recording off differs slightly from most other recording software;
It is not possible to record an input without Audacity also recording the inputs that come before this. For example, when recording the signal coming into input 6 of your interface, Audacity requires you to record inputs 1,2,3,4,5 and 6.